The present invention relates to a color image forming apparatus and an image forming system applicable preferably to a color image forming apparatus, a facsimile machine thereof, a digital copying machine thereof and a multifunctional machine thereof.
In recent years, a tandem type color printer, a color copying machine and a multifunctional machine thereof have come into widespread use. The color image forming apparatus is equipped with various types of image writing unit for yellow (Y), magenta (M), cyan (C) and black (BK), development means, photoconductor drum, intermediate transfer belt and fixing apparatus.
In the Y-color image writing unit, for example, an electrostatic latent image is generated on the photoconductor drum based on the Y-color image data. The development means attaches Y-color toner to the electrostatic latent image generated on the photoconductor drum to form a color toner image. The photoconductor drum transfers the toner image onto the intermediate transfer belt. The same processing is applied to other colors such as M, C and BK. The color toner image transferred onto the intermediate transfer belt is transferred onto paper and is then fixed in position by a fixing apparatus.
The color image forming apparatus of this type that allows formation of color image on both sides of paper is also developed and manufactured. The duplex image forming function is used to create a booklet, for example. Paper thicker than the paper used in the book is often used as the paper for the front cover and back cover. Subsequent to duplex image formation, the paper for the front and back covers is subjected to double folding and stapling. In such a process of duplex image formation, the paper is known to shrink after an image has been formed on one side of paper. This is because the paper with a color toner image transferred thereon is heat-shrunk by the process of fixing. The degree of shrinkage is more serious for thicker paper. Thus, to registrate the image positions of the paper front and rear surfaces, the image forming conditions must be modified.
In connection with the image writing unit of this type, Patent Document 1 discloses a color image forming apparatus. This color image forming apparatus is provided with a change means for controlling the change of a clock frequency. When the image forming operation is shifted from one side of the paper to the other side, this change means controls the change of a pixel clock frequency for controlling a laser drive circuit and a drive clock frequency for controlling a polygon motor. Provision of this change means allows the image sizes on the front and rear to be agreed.
FIGS. 23(A) through (D) are time charts representing the operation example (based on BK color) during image formation in the prior art image writing units for Y, M, C and BK colors.
For example, assume that an instruction is given to start the front surface image formation at time t1′ during duplex image formation in the image writing units or Y, M, C and BK colors. Then the Y-color sub-scanning valid area signal (hereinafter referred to as YVV) rises at time t2′ in FIG. 23(A). The time when the YVV signal stays on a high level (hereinafter referred to as H level) indicates the time duration of Y-color image formation (in progress on the front surface of paper).
Further, the M-sub-scanning valid area signal (hereinafter referred to as MVV) shown in FIG. 23(B) rises at time t3′. The H level period of the MVV signal indicates the time duration of M-image formation (in progress on the front surface of paper). The C-sub-scanning valid area signal (hereinafter referred to as CVV) rises at time t4′ shown in FIG. 23(C). The H level period of the CVV signal indicates the time duration of C-image formation (in progress on the front surface of paper).
When the YVV signal falls to a low level (hereinafter referred to as L level) at time t5′, processing of Y-color image formation terminates. Further, the BK-sub-scanning valid area signal (hereinafter referred to as KVV) rises at time t6′ shown in FIG. 23(D). The H level period of the KVV signal indicates the time duration of K-image formation (in progress on the front surface of paper).
When the MVV signal falls to a low level (hereinafter referred to as L level) at time t7′, processing of M-image formation terminates. Further, when the CVV signal falls to a low level (hereinafter referred to as L level) at time t8′, processing of C-image formation terminates. If the KVV signal falls to a low level (hereinafter referred to as L level) at time t9, processing of BK-image formation terminates.
At time t10′, the BK-image writing unit starts control of changing the rotary speed (scanning speed) of the polygon mirror. The control of changing the polygon mirror rotary speed in the Y, M, C and BK color image writing units is carried out prior to control of the BK color rotary speed. It is performed after termination of Y, M, C and BK color image formation. Further, Y, M, C and BK color polygon mirror surface phase control is carried out in conformity to the BK-color main scanning reference signal. Thus, it is performed after rotary speed control of changing the BK-color rotary speed has terminated and the rotary speed has been stabilized.
Assume that an instruction is given to start rear surface image formation upon completion of the surface phase control such as change of the rotary speed in the BK-color image writing unit and change of the Y, M and C color polygon mirror surface phase, for example, at time t11′ after the lapse of PLL lock wait time Tε. Then the Y-color YVV signal rises at time t12′ shown in FIG. 23(A). The H level period of the YVV signal indicates the time duration of Y-color image formation (in progress on the rear surface).
After that, the M-color MVV signal rises at time t13′ as shown in FIGS. 23(B) through (D). Then the BK-color KVV signal rises at time 16′ subsequent to fall of the YVV signal at time t15′. The timed interval of the generation of such a sub-scanning valid area signal is controlled based on the main scanning reference signal (hereinafter referred to as index signal) of each color. The index signal is a signal obtained by detecting the optical beam reflected from the polygon mirror.
Incidentally, the PLL lock wait time Tε shown in FIG. 23(D) is necessary to control the rotary speed of the final BK-color polygon mirror for the rear surface of paper and the Y, M and C color polygon mirror surface phase. It is the time before the BK-color polygon mirror rotation is stabilized subsequent to change of the rotary speed and surface phase. The control of changing the rotary speed and surface phase of this type is necessary to change the timed interval for image formation, since the size of paper in image formation on the paper rear surface is slighted reduced as compared to the size of paper in image formation on the paper front surface (refer to Patent Document 1).
FIGS. 24(A) and (B) are explanatory diagrams showing an example of paper shrinking in the duplex image formation mode. Paper P shown in FIG. 24(A) is in the state just before being fixed, after a color toner image has been secondarily transferred thereon. The paper P has a length of L mm and a width of W mm. The paper P′ shown in FIG. 24(B) is in the state after the paper P has been fixed. The length of the paper P′ has been reduced to L′ mm, and the width to W′ mm. The paper size is considered to be reduced by evaporation of moisture. The image on the rear surface must be formed in response to such a reduced size of the paper P. Incidentally, the image formation conditions must conform to the paper size subsequent to shrinkage, L′ mm×W′ mm. Otherwise, the image formation positions (sizes) of the front and rear surfaces will be misaligned.
In response to such a reduced size of the paper P, the frequency of the polygon motor drive clock (hereinafter referred to as CLK) is changed. Assume that the polygon drive CLK frequency before shrinkage, viz., during image formation on the front surface is F0, and the polygon drive CLK frequency after shrinkage, viz., during image formation on the rear surface is F. Then F=F0×L/L′ is set.
Further, the pixel CLK frequency for controlling the laser beam is changed. Assume that the pixel CLK frequency prior to shrinkage is f0, and the pixel CLK frequency subsequent to shrinkage is f. Then f=(L/L′)×(W/W′)×f0 is set. As described above, in response to the shrinkage of the paper P, the polygon drive CLK frequency and pixel CLK frequency are changed, whereby an image accurately registered on the front and rear surfaces can be obtained.
Assume that the process linear speed prior to shrinkage is V0, process-to-process gap prior to shrinkage is G0, distance between units is process gap G and process lineal speed is V. When the polygon drive CLK frequency has been changed from F0 to F, then:
1. The apparent process linear speed V will be changed to V0×F0/F=V0×L′/L.
2. The process-to-process gap G (pixel) will be changed to G0×V0/V=G0 ×L/L′.
As described above, the process linear speed V will undergo a change. Thus, the amount of correcting the color misalignment corresponding to process-to-process gap G requires correction of the speed amount of the front-to-rear magnification change. Accordingly, if polygon mirror surface phase adjustment function is provided, the surface phase is controlled when front/rear switching is performed. The aforementioned polygon mirror rotary speed and polygon mirror surface phase for Y, M and C colors is carried out are controlled at the time of duplex image formation as well as tray switching. Here the process linear speed corresponds to the rotary speed of the photoconductor as an image forming member for image formation.
FIGS. 25(A) through (H) are time charts representing examples of image formation (BK-color as a reference) in an Y, M, C and BK color image writing unit according to the prior art at the time of tray switching.
After the leading edge of the paper fed out of the tray 1 has been detected by the leading edge sensor (not illustrated), the signal VTOP given in FIG. 25(A) rises at time T11′ synchronized with the BK-color index signal (hereinafter referred to as KIDX signal) shown in FIG. 25(H). At the YVV signal start timing shown in FIG. 25(B), the KIDX counter (not illustrated) starts to count the pulse of the KIDX signal and the YVV signal rises at time T12′ synchronized with the KIDX signal. The YVV signal shown in FIG. 25(C) rises at time T13′ synchronized with Y-color index signal (hereinafter referred to as YIDX signal) shown in FIG. 25(D). When this YVV signal stays on the H level, a Y-color image is formed on the paper from the tray 1. Upon image formation, the Y-color polygon mirror rotary speed is controlled. The YIDX signal frequency fluctuates when the speed change is controlled.
In the similar manner, an M-color image is formed on the paper from the tray 1 when the MVV signal shown in FIG. 25(E) stays on the H level. Upon image formation, the polygon mirror rotary speed is controlled. A C-color image is formed on the paper from the tray 1 when the CVV signal shown in FIG. 25(F) stays on the H level. Upon image formation, the polygon mirror rotary speed is controlled.
The KVV signal shown in FIG. 25(G) rises at time T14′ synchronized with the KIDX signal shown in FIG. 25(H). A BK-color image is formed on the paper from the tray 1 when the KVV signal stays on the H level. Upon image formation, the BK-color polygon mirror rotary speed is controlled. The KIDX signal frequency fluctuates when the speed change is controlled.
In the example of image formation at the time of tray switching, the surface phase of Y, M and C color polygon mirror is controlled based on the KIDX signal. Accordingly, this control starts subsequent to completion of the BK-color polygon mirror rotary speed control. Image formation on the paper from tray 2 starts subsequent to control of the Y-color polygon mirror surface phase control.
As described above, in the example of image formation at the time of tray switching using the BK color as a reference, the phase cannot be changed until the BK-color speed change is completed. Further, image formation on the next sheet of paper (fed from the tray 2) cannot be started until the Y-color phase change is completed.
A light beam scanning apparatus is shown in the Patent Document 2. This light beam scanning apparatus is provided with: a rotary reference signal generation means for generating a rotary reference signal corresponding to each polygon mirror; and a phase control means for controlling the rotary reference signal generation means in such a way that the light beam detection signals corresponding to the polygon mirrors will be placed in a desired positional relationship, with reference to the light beam detection signal corresponding to the reference polygon mirror. Such a phase control means corrects the color misalignment of less than one scan pitch.
The Patent Document 3 discloses a laser beam scanning apparatus. This laser beam scanning apparatus is provided with a rotary phase control means that calculates the time difference between the light beam detection signal corresponding to the reference polygon mirror and the light beam detection signals corresponding to the remaining polygon mirrors; and compares the phase control data based on this time difference with the phase control data corresponding to the reference polygon mirror, thereby generating a rotary frequency. Such a rotary phase control means provides a simple control of the direction of the mirror surface of the polygon mirror.
Patent Document 1 represents Official Gazette of Japanese Patent Tokkai 2003-0262991 (page 3 and FIG. 3), Patent Document 2 represents Official Gazette of Japanese Patent Tokkaihei 15-3452166 (page 7 and FIG. 2), and Patent Document 3 represents Official Gazette of Japanese Patent Tokkaihei 15-3458878 (page 5 and FIG. 1).
It should be noted that the prior art color image forming apparatus has the following problems.
(i). In the color image forming apparatus disclosed in the Patent Document 1, the example of the operations at the time of duplex image formation shown in FIGS. 23(A) through (D) (wherein BK color is used as a reference) shows that the image formation timing is determined by the signal (non-pseudo index signal) obtained by detecting the light beam reflected by the BK-color polygon mirror, whereby the YVV signal, MVV signal, CVV signal and KVV signal are generated. Accordingly, in the duplex image formation mode, image formation signal for starting image formation on the next page cannot be issued until the final BK-color image formation on the front paper is completed. In the example given in FIG. 23(D), the image formation start signal for starting image formation on the reserve surface of paper can be issued only after the lapse of the PLL lock wait time Tε required to control the rotary speed of the final BK-color polygon mirror for the front page of the paper and the Y, M and C color polygon mirror surface phase. This PLL lock wait time adversely affects the productivity of the color image forming apparatus.
(ii). The aforementioned problem is found also in the example of image formation operation at the time of tray switching shown in FIGS. 25(A) through (G). Namely, phase cannot be changed until the BK-color speed change is completed and the KIDX is stabilized. Further, image formation on the sheets of paper fed out of the tray 2 cannot be started until Y-color speed change is completed and the YIDX is stabilized.
(iii). When a color image forming apparatus is configured using the laser beam apparatus disclosed in Patent Documents 2 and 3, any one of a plurality of image writing units is regarded as a reference unit, and the control means controls the surface phase of the polygon mirror corresponding to that signal. Thus, the surface phase control can be started only after the lapse of time required for stabilization of the index signal corresponding to the reference polygon mirror.
(iv). Further, it is necessary to use a color tandem machine to adjust the speed of the polygon mirror and to regulate the apparent process linear speed. The surface phase in addition to the rotary speed adjustment of the polygon mirror must be controlled at the time of registering the front and rear surfaces. This arrangement adversely affects the productivity.
(v). One way to solve these problems is as follows: Based on the index signal of the color for the first image formation on this sheet of paper, the timed interval for image formation in the remaining M, C and BK colors is determined, and the paper from the next tray 2 is fed out during image formation in other colors on this sheet, thereby starting image formation in the first color on this sheet. This method also produces a time lag, which adversely affects the productivity of the color image forming apparatus.
FIGS. 26(A) through (H) are time charts representing the examples of operations (wherein Y color is used as a reference) related to the improved method (proposal) of image formation timing in a color image forming apparatus.
The leading edge of the paper fed out of the tray 1 and the VTOP signal shown in FIG. 26(A) rises at time T21′ synchronized with the YIDX signal shown in FIG. 26(D). The YIDX counter (not illustrated) is activated, YIDX signal pulses are counted, and the YVV start timing signal shown in FIG. 26(B) rises at time T22′ synchronized with the YIDX signal. The YVV start timing signal shown in FIG. 26(C) rises at time T23′ synchronized with the YIDX signal shown in FIG. 26(D). When the YVV signal stays on the H level, Y-color image formation is applied to the paper from the tray 1.
The rotary speed of the Y-color polygon mirror is controlled after the KVV signal shown in FIG. 26(H) has risen. This is because image formation timing in each color is based on the Y color. During the speed change control, the YIDX signal frequency fluctuates. M-color image formation on the paper from the tray 1 is carried out when the MVV signal shown in FIG. 26(E) stays on the H level. Upon completion of this formation, the rotary speed of the M-color polygon mirror and phase change thereof are controlled. C-color image formation on that paper is carried out when the CVV signal shown in FIG. 26(F) remains on the H level. Upon completion of this formation, the rotary speed of the C-color polygon mirror and phase change thereof are controlled.
The KIDX counter (not illustrated) is activated, YIDX signal pulses are counted, and the KVV start timing signal shown in FIG. 26(G) rises at time T24′ synchronized with the YIDX signal. The KVV start timing signal shown in FIG. 26(H) rises at time T25′ synchronized with the YIDX signal shown in FIG. 26(I). When the KVV signal stays on the H level, BK-color image formation is applied to the paper from the tray 1. Upon completion of this formation, the rotary speed of the BK-color polygon mirror and surface-phase thereof are controlled. During the speed and phase change control, the KIDX signal frequency fluctuates.
The example of the image formation operation at the time of tray switching provides an advantage in that the Y-color image formation onto the paper from the tray 2 can be started before completion of the image formation onto the paper from the tray 1. The image formation speed can be increased by the amount corresponding to this advantage as compared to the Patent Document 1. However, in this improved version of image formation startup timing method (proposal), a YIDX signal is used to determine the BK-color KVV start timing. Accordingly, the Y-color polygon mirror rotary speed control cannot be initiated before the BK-color KVV start timing is established and the KVV signal rises. In FIG. 26(C), time lag TL is defined as the time from the completion of Y-color image formation to the rise of the BK-color KVV signal shown in FIG. 26(H). Thus, the time lag TL may have an adverse effect on the productivity of the color image forming apparatus.